For the new study, Verberk and colleague David Bilton instead focused on how varying oxygen levels affect stonefly larvae, which, like dragonflies, live in water before becoming terrestrial adults. Higher concentrations of oxygen in air would have meant higher concentrations dissolved in water.

The results showed that juvenile stoneflies are more sensitive to oxygen fluctuations than their adult counterparts living on land.

This may be because insect larvae typically absorb oxygen directly through their skin, so they have little or no control over exactly how much of the gas they take in. By contrast, adult insects can regulate their oxygen intake by opening or closing valve-like holes in their bodies called spiracles.

While crucial for life, oxygen can be poisonous in large quantities: Humans exposed to excess oxygen can suffer cell damage leading to vision problems, difficulty breathing, nausea, and convulsions.

The new theory could also explain why giant insects continued to exist even after Earth’s atmospheric oxygen levels began decreasing, he said.

"If oxygen actively drove increases in body mass to avoid toxicity, lower levels would not be immediately fatal, although in time, they will probably diminish performance of the larger insects," since adults would have evolved to require more oxygen and would get sluggish in air with lower levels, Verberk said.

"Such reduced performance will eventually have made it possible for other species to outcompete the giants."